Fruit color is an important quality factor in red table and wine grapes. Commercial harvest requires sufficient levels of color in commercially mature fruit and this can be a significant challenge for table grape growers. Fruit color development can be influenced by a number of factors including the grape cultivar, rootstock, plant vigor, climate, canopy management, light exposure, crop load, irrigation, fertilization, and plant growth regulators. Consequently, achieving optimal fruit color requires a programmatic approach rather than the use of a single tool or practice.
The plant growth regulator ethephon, an ethylene-releasing chemical, is one tool that can be used to help improve color development (Jensen et al., 1975; Szyjewicz et al., 1984). However, ethephon has shortcomings and risks. For example, in Crimson Seedless table grapes, ethephon efficacy is often inconsistent or poor. Multiple applications of high rates of ethephon may be required in order to achieve the desired level of coloration, if it can even be achieved. In addition to being inconsistent in its coloring effect, ethephon can cause berry softening, berry cracking or splitting and poor storage and shelf life (e.g. Jensen et al., 1975; Szyjewicz et al., 1984). Because of the importance of fruit color development, there is a need for additional tools to help improve grape coloration.
S-Abscisic acid (S-ABA) is a naturally occurring plant hormone found in all higher plants (Cutler and Krochko, 1999. Finkelstein and Rock, 2002). Levels of S-ABA in plants range from a few parts per billion in some aquatic plants to 10 parts per million in avocado fruit mesocarp (Milborrow, 1984). S-ABA is involved in many major processes during plant growth and development including dormancy, germination, bud break, flowering, fruit set, general growth and development, stress tolerance, ripening, maturation, organ abscission, and senescence. S-ABA also plays an important role in plant tolerance to environmental stresses, such as drought, cold, and excessive salinity.
One key role of S-ABA in regulating physiological responses of plants is to act as a signal of reduced water availability to reduce water loss, inhibit growth and induce adaptive responses. All these functions are related to stomatal closure of plant leaves (Raschke and Hedrich, 1985). When stomata close, plants conserve water to survive in environmental stresses. However, stomatal closure also results in the reduction of photosynthesis, and thus growth. Stomatal closure is a rapid response of plants to S-ABA. The mechanism of S-ABA that causes stomatal closure has been studied, and the effect has been shown to be primarily due to the effect of S-ABA on guard cell ion channels. Specifically, S-ABA blocks H+ efflux from and K+ influx into guard cells and promotes K+, Cl−, and malate efflux and Ca2+ influx. The net effect of S-ABA is to reduce the total osmotica in the guard cells, which in turn decreases the water content in the cell. This causes the guard cells to lose their turgor and thus close the stomata (Assmann 2004). The closing of stomata results in reduced transpiration of the plant leaf. In grapes, application of S-ABA has been reported to increase stomatal resistance in grapevines, thereby reducing the gas exchange and stomatal transpiration of the leaves (Düring and Broquedis, 1980).
The exogenous application of S-ABA to red grapes prior to harvest has been shown to increase and accelerate the accumulation of anthocyanins and increase the red color of the grape berry skins (e.g. Han et al., 1996; Lee et al., 1997; Kondo et al., 1998; Pepe et al., 2006). S-ABA has been shown to be effective on red color varieties on which ethephon often is not commercially effective (e.g. Crimson Seedless). At very high rates, S-ABA may be associated with softened, split berries, or leaf yellowing.
The application of the plant growth hormones/plant growth regulators S-ABA and ethephon both stimulate development of red color by increased accumulation of anthocyanin pigment in the berries.
While both S-ABA and ethephon stimulate development of color, there are potential shortcomings with each material. Ethephon performance can be inconsistent, dependent upon such factors as cultivar, vineyard, year, and environmental conditions. High levels of ethephon are known to cause problems such as berry softening, berry splitting, and short shelf life (Jensen et al, 1975; Szyjewicz et al., 1984). S-ABA has not been registered and commercialized. Results from S-ABA field experiments show good efficacy, but response can sometimes be variable.